Rx interband carrier aggregation is required by some mobile phone standards, e.g., by LTE-advanced. Rx interband carrier aggregation requires two Rx paths at different frequency bands to be active simultaneously, thereby providing a higher downlink data rate. Thus, carrier aggregation increases the reception bandwidth.
Moreover, standards like LTE and WCDMA and their cellular front end implementations set stringent requirements for the Tx to Rx isolation of a duplexer in order for the own Tx not to desensitize the corresponding Rx port. Furthermore, a cellular phone can only support carrier aggregation, if the same Tx to Rx isolation requirement is expanded also to the isolation between the main Tx port and the aggregated Rx port.
As an example, a situation is considered wherein the main operating band with both Tx port and Rx port is band 2 and the aggregated band is band 4. In this case it is equally important to provide a good isolation from the band 2 Tx port to band 2 Rx port and to provide a good isolation from the band 2 Tx port to band 4 Rx port.
In particular, for band pairs that are located close to each other in frequency, this problem cannot be solved by connecting the antenna with a diplexer because a diplexer cannot sufficiently separate frequencies in bands that are close to each other. Such band pairs are formed, for example, by bands 8 and 20 or by bands 2 and 4.
Carrier aggregation of closely spaced bands may be supported by a combination of two duplexers at the antenna port with some additional matching networks. Two duplexers combined at the antenna port and optionally with some matching structures are also referred to as a quadplexer.
However, this approach requires the redesign of the acoustic part of the duplexers so that each of the Tx parts provides a good attenuation at the two different Rx frequencies. This requires a compromise with some other parameters, e.g., Tx insertion loss, due to additional resonators or other structures in the Tx filter part of the duplexer. Thus, it is possible to realize a quadplexer with an acceptable Tx to Rx isolation. However, the Tx to aggregated Rx isolation of this quadplexer is not sufficient.